mirror of https://github.com/abinit/abinit.git
54 lines
2.6 KiB
Markdown
54 lines
2.6 KiB
Markdown
---
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description: How to specify bands and occupation numbers, for metals or insulators
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authors: FJ
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---
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<!--- This is the source file for this topics. Can be edited. -->
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This page gives hints on how to specify bands and occupation numbers,
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for metals or insulators with the ABINIT package.
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## Introduction
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Metallic as well as insulating systems can be treated, depending on the value
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of [[occopt]]. The default value of [[occopt]] corresponds to an insulator (or
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finite molecule): the number of bands (or states for a molecule) is deduced
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from the number of electrons brought by each pseudopotential ion, and then all
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the bands are occupied (by two electrons in case of a non-spin-polarized
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system, or by one electron in the case of a spin-polarized system), and a small
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number of empty bands are added, e.g. to obtain the band gap.
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For a metallic system, use a value of [[occopt]] between 3 and 7. ABINIT will
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compute a default number of bands, including some nearly unoccupied ones, and
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find the occupation numbers. The different values of [[occopt]] correspond to
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different smearing schemes (smearning defined by [[tsmear]] for defining the
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occupation numbers, e.g. Fermi broadening, the Gaussian broadening, the
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Gaussian-Hermite broadening, as well as the modifications proposed by Marzari.
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Finite temperatures can also be treated thanks to a smearing scheme
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([[cite:Verstraete2002]] scheme) using [[tphysel]].
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For gapped materials, the treatment of quasi-Fermi energies in the valence and conduction band,
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with populations of electrons (in the conduction bands) and holes (in the valence bands)
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is available with [[occopt]]=9.
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It is possible to define manually the number of bands (input variable
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[[nband]]) as well as the occupation numbers (input variable [[occ]]). This
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might be useful to perform a Δ-SCF calculation for excited states.
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## Related Input Variables
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{{ related_variables }}
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## Selected Input Files
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{{ selected_input_files }}
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## Tutorials
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* [[tutorial:base1|The tutorial 1]] deals with the H2 molecule: get the total energy, the electronic energies, the charge density, the bond length, the atomisation energy
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* [[tutorial:base2|The tutorial 2]] deals again with the H2 molecule: convergence studies, LDA versus GGA
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* [[tutorial:base3|The tutorial 3]] deals with crystalline silicon (an insulator): the definition of a k-point grid, the smearing of the cut-off energy, the computation of a band structure, and again, convergence studies ...
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* [[tutorial:base4|The tutorial 4]] deals with crystalline aluminum (a metal), and its surface: occupation numbers, smearing the Fermi-Dirac distribution, the surface energy, and again, convergence studies ...
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